The hydraulic press depends on Pascal's principle: The
pressure throughout a closed system is constant. At one end of the system
is a small area piston driven by a lever to increase the mechanical advantage.
The small-diameter tubing leads to the other end of the system. This is
a large area piston that exerts a large force equal to the force exerted
on the small piston, multiplied by the ratio of the areas.

The hydraulic press was made practical by Joseph Bramah's
1796 development of an oil impregnated leather gasket that sealed the large-diameter
piston.

This demonstration press is in the collection of the United
States Military Academy at West Point, and, in my opinion, is one
of the best pieces of the collection. Although the floral decorations on
the top and bottom are well-preserved, it has no maker's name.

When I saw this glass demonstration apparatus at the University
of Cincinnati in May 2001, I thought that it was a demonstration pump.
I finally found it in the 1929 catalogue of the Chicago Apparatus Company
and I was partly correct. It is really a Model Hydraulic Press, and sold
for $2.25. What is missing is the glass cylinder in the left-hand piston.
The space in the board is to allow this piston to rise a short distance
as the plunger was depressed. The vertical glass tube in the middle is
the one-way valve that permits the left-hand piston to rise, but not fall.

This apparatus, which is only 24 cm high, was probably
mounted on the board to allow the operation of the press to be seen by
a large class using shadow projection.

The demonstration hydraulic press at the left is in the
Millington/Barnard Collection at the University Museum at the University
of Mississippi in Oxford. It was bought by Frederick A.P. Barnard, the
second Professor of Natural College at the University, in the second half
of the 1850s from Lerebours et Secretan of Paris. The cost was 250 francs
or $50.

The apparatus is incomplete. On the right-hand side is
the pumping mechanism, consisting of a cylinder and piston, with a pivoted
handle used to gain mechanical advantage.